The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have generated generations of ICs, wherein each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometric size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing. For these advances to be realized, similar developments in IC processing and manufacturing are needed. For example, the need to perform higher resolution lithography processes grows. One lithography technique is extreme ultraviolet lithography (EUVL).
The EUVL employs scanners using light in the extreme ultraviolet (EUV) region, having a wavelength of about 1 nm to about 100 nm. The EUVL is very similar to common optical lithography in that it needs a mask to print wafers, except that it employs light in the EUV region, e.g., at 13.5 nm. At the wavelength of 13.5 nm or so on, all materials are highly absorbing. Thus, the EUV scanners use reflective optics rather than refractive optics, i.e. mirrors instead of lens and reflective masks are used. The EUV scanners provide the desired circuit pattern on an absorption layer (EUV mask absorber) formed on a reflective mask. A multi-layered (ML) structure is used as a EUV mask blank. Moreover, EUV masks typically require a pellicle membrane, which serves as a protective cover to protect the EUV mask from damage and/or contaminant particles.
Although existing EUV lithography systems and processes have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.